In order to manage the movement of a plurality of unmanned vehicles, unmanned dump trucks for example, for transporting soil in a wide working site, such as a quarry and mine, a monitoring station is established as a ground station and a vehicle monitoring system is structured such that this monitoring station manages and monitors these unmanned vehicles comprehensively.
This vehicle monitoring system has a transmission/reception equipment (e.g. VHF system) for performing long distance wireless communication between the monitoring station and the plurality of vehicles, and various data, including the position data of the respective vehicles which were measured, is transmitted to the monitoring station in extremely short cycles (e.g. every one second), so that the monitoring station can monitor each vehicle knowing the accurate position of the respective vehicles.
The monitoring station which received the position data from a vehicle transmits data to notify reception so that each vehicle can confirm the occurrence of failure of its transmission/reception equipment.
Recently, however, it is becoming necessary to monitor many vehicles (50-100 vehicles) which travel very long distances (approx. 10 km) in many traveling courses, where information to be handled is dramatically increasing.
To handle this status, it is necessary to install transmission/reception equipment which can perform fast wireless communication in a wide range (long distance).
According to currently available technology, the following are the two types of communication systems which can practically support such monitoring of vehicles.
1) VHF, UHF PA1 2) SS (spread spectrum system) wireless communication
However, if the 1) VHF or UHF system is applied to the above mentioned vehicle monitoring system, the communication system, which allows long distance communication (10 km-20 km), can provide communication in the whole region of a wide working site, but the current positions of many vehicles cannot be constantly known since the communication speed is slow (9600 bps). In other words, a large volume of data is transmitted from many vehicles to the monitoring station. And since the communication system with a slow communication speed handles this large volume of communication information, communication lines jam, the load on the communication lines increases, and the management and monitoring of the vehicles become virtually impossible.
If the 2) SS wireless communication system is applied to the vehicle monitoring system, the communication system which allows high-speed communication (256 Kbps) can transmit an extremely large volume of information at high-speed, but cannot provide communication in the whole region of a wide working site, which is now becoming increasingly wider, since the propagation distance of radio waves is short (100 m-1 Km).
Also, in order to provide communication in the whole region of a wide working site by SS wireless communication, such auxiliary equipment as radio stations must be installed at various locations of the working site to compensate for the insufficient propagation distance of radio waves. This increases cost for initial investment and maintenance, which lessens the practicality of this system.
Conventionally the communication system in the above 1) has been adopted, and in order to compensate for the management of vehicles performed by the monitoring station, an obstacle sensor is installed on each vehicle, so that this sensor confirms the presence of other vehicles to prevent collision. However such a system which prevents collision by such a sensor alone has safety problems, and is not desirable. This is because 100% collision cannot be prevented when many vehicles pass cross sections or pass another vehicle.
In both the communication systems of the above 1) and 2), the monitoring station controls all vehicles, therefore the problem of excessive load on the monitoring station remains unsolved.
Further, when many vehicles transmit data to the monitoring station, the monitoring station transmits data for notifying the reception of the data back to the many vehicles so that each vehicle confirms the failure of the transmission/reception equipment of the respective vehicle, as mentioned above, but if this method is implemented by the communication method in the above 1), the monitoring station cannot always transmit the data for notifying the reception of the data back to many vehicles due to the slow communication speed of the system, and as a result, the vehicles cannot quickly and accurately confirm the failure of the respective vehicles.
In this way, the number of vehicles which a conventional monitoring system can manage is limited because of the shortcomings of the communication system, even though it is necessary to exchange a large volume of data.
With the foregoing in view, it is an object of the present invention to provide means to perform communication over the whole region of a wide working site without increasing cost due to the installation of auxiliary equipment, and to sufficiently perform mutual control of vehicles with a light burden on a monitoring station without sacrificing safety, and to quickly and accurately confirm failures which occur to transmission/reception equipment, so that an abnormality can be dealt with quickly and properly.